Preparation of phosphorus and nitrogen containing waterborne polyurethane and its application in polyester fabrics for flame retardant finishing
Objective Polyester fabric has many advantages,including good thermal stability,chemical resistance and excellent mechanical properties,which make it widely used in clothing,furnishings and packaging.However,polyester fabric has a low limiting oxygen index and is flammable.When it burns,the polyester fabric produces a large amount of molten droplets which could quickly spread the flames to other combustible materials,and releases a significant volume of toxic smoke.Statistics show that most casualties in fires are caused by smoke inhalation,with more than one-third of total fire casualties directly attributable to smoke poisoning.Therefore,it is crucial to modify polyester fabric to control the release of smoke as well as the formation of molten droplets during combustion.Method The Schiff base derivative 4-(((4-hydroxyphenyl)imino)methyl)benzene-1,3-diol(HIMB)was prepared and reacted with 9,10-dihydro-9-oxo-10-phosphophenanthrene-10 oxide(DOPO)to obtain a phosphorus-and nitrogen-containing Schiff base derivative flame retardant 4-DOPO-(((4-hydroxyphenyl)amino)methyl)benzene 1,3-diol(DOPO-HAMB).DOPO-HAMB was used as a chain extender to introduce into the molecular main chain of waterborne polyurethane(WPU)to prepare inherently flame-retardant waterborne poly-urethane(FRWPU).FRWPU was then applied to the surface of polyester fabrics through an impregnation method,providing the polyester fabrics with flame retardant properties while ensuring mechanical performance.Results Photos and scanning electron microscopy images of the actual polyester fabrics before and after the treatment showed that the treated polyester fabric had a slight yellowing compared to the original polyester fabric,with no significant change in transparency.Scanning electron microscopy images showed that the surface of the treated polyester fabric was coated and the gaps between fibers were almost filled.Thermogravimetric experiments demonstrated that the temperature at 5%thermal decomposition,the temperature at the maximum rate of decomposition,and the char yield at 650℃of the treated polyester fabric were all lower than that of the raw polyester fabric.The results of the limiting oxygen index(LOI)and vertical burning tests indicated that the raw polyester fabric had an LOI value of only 17.1%,the WPU treated fabric's LOI was 19.2%,and when DOPO-HAMB was introduced into the WPU molecular main chain,the LOI value of the FRWPU12(the mass percentage of DOPO-HAMB in WPU is 12%)treated fabric increased to 24.5%,which was 7.4%higher than that of the raw polyester fabric.The raw polyester fabric continued to burn for 18 s after ignition,with a damage length reaching 30 cm,producing dripping that ignites the cotton beneath.However,the FRWPU12 treated fabric did not continue burning and was self-extinguished immediately after removing from the flame,with a damage length of only 9.7 cm and no dripping during combustion.Scanning electron microscopy images and energy dispersive X-ray spectroscopy spectra of the char residue showed that the surface of the char from the raw polyester fabric was fragmented and contained only C and O elements,while the char from the FRWPU3(the mass percentage of DOPO-HAMB in WPU is 3%)and FRWPU12 treated polyester fabrics was smooth and dense,containing P in addition to C and O,indicating the principal role of P from DOPO-HAMB in the WPU molecular main chain was in the condensed phase.Tensile test of the polyester fabric before and after treatment revealed that the warp and weft breaking strength of the raw polyester fabric were 508 N and 253 N,and the warp and weft elongations at break were 34.0%and 16.9%,respectively.After treatment,the maximum warp breaking strength of the polyester fabric increased to 521 N,and the maximum weft breaking strength to 567 N,with the highest weft breaking at break up to 23.8%.The increase in warp and weft breaking strengths are attributable to the WPU and FRWPU between adjacent fibers,which increased the tightness of the yarns and the connection strength between the yarns.In the water-fastness test,the FRWPU12 treated fabric showed a significantly higher LOI value than the raw polyester fabric even after 50 washing cyeles with water,indicating good water washability of FRWPU12 on the polyester fabric surface.Conclusion A phosphorus-and nitrogen-containing Schiff base derivative flame retardant,DOPO-HAMB,was synthesized and introduced into the main molecular chain of WPU as a chain extender to obtain FRWPU.The WPU and FRWPU were then applied to the surface of the polyester fabric by an impregnation method.When the mass percentage of DOPO-HAMB in WPU is 12%,the LOI value of the treated fabric reaches 24.5%,the length of damage in the vertical burning test is significantly decreases,and no dripping occurs during combustion.Compared with the raw polyester fabric,the warp and weft tensile breaking strength of the treated polyester fabric are improved to a certain extent.Moreover,the LOI value of the treated polyester fabric after 50 washing cycles remains significantly higher than that of the raw polyester fabric,demonstrating good water washability.
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